The heart is the first organ to form during embryogenesis, and abnormalities in this process result in congenital heart diseases, the most common cause of birth defects in humans (Non-Patent References 1 and 2). Molecules that mediate cardiogenesis are of particular interest because of their potential use for cardiac regeneration (Non-Patent References 3 and 4).
Previous studies have shown that soluble factors such as Wnts, Wnt inhibitors, bone morphogenetic proteins (hereinafter, abbreviated as BMPs), and fibroblast growth factors (hereinafter, abbreviated as FGFs) mediate tissue interactions that are crucial for cardiomyocyte specification (Non-Patent References 2 and 4).
Wnts are proteins that control morphogenesis, and are known to be involved in various phenomena such as development, stem cell differentiation control, and cell malignant transformation. Further, there are reports that Wnts are important factors for the growth regulation and survival of stem cells (Non-Patent References 5 and 6).
It is known that Wnts bind to cell membrane receptors and transmit intracellular signals via at least three kinds of pathways, to thereby express their actions. Known as the cell membrane receptors to which Wnts bind are Frizzled (sometimes abbreviated as Frz) as a seven-pass transmembrane receptor and low-density lipoprotein receptor-related proteins 5 and 6 (sometimes abbreviated as LRP5 and LRP6, respectively) as single-pass transmembrane receptors (Non-Patent References 7 and 8). It is conceivable that there are at least three kinds of pathways in signalling pathways to be controlled by Wnts, i.e., a β-catenin pathway, a planar cell polarity (PCP) pathway, and a calcium ion (Ca2+) pathway. The β-catenin pathway has been known for many years and is also called a canonical pathway. The pathway is characterized by the stabilization of cytoplasmic β-catenin that plays an important role in the transmission of Wnt signals to the nucleus. The abnormal activation of the pathway is thought to be associated with oncogenesis. On the other hand, the PCP pathway and the Ca2+ pathway are called non-canonical pathways. The PCP pathway is characterized by the activation of a low molecular weight G protein Rho and Jun kinase belonging to the MAP kinase family. Further, the Ca2+ pathway is characterized by the activation of downstream protein kinases, protein kinase C (PKC) and calmodulin kinase, through an increase in intracellular calcium concentration.
IGFBPs are proteins that bind to insulin-like growth factors (hereinafter, abbreviated as IGFs) to modulate the actions of the factors. It has been made clear that there are two molecular species in IGFs, which are called IGF-I and IGF-II, respectively. IGF-I and IGF-II, each of which is structurally highly similar to insulin, bind to the corresponding cell surface receptors, a type I IGF receptor and a type II IGF receptor, and play an important role in the proliferation and differentiation of various cells.
It has been made clear that there are six molecular species in IGFBPs, which are called IGFBP-1 to IGFBP-6, and those species are widely expressed in various tissues. Any of IGFBP-1 to IGFBP-6 binds to IGFs to modulate interactions between the IGFs and their receptors, to thereby control the actions of the IGFs.
Meanwhile, there are reports that several actions of IGFBPs are independent of IGFs. However, there are little findings on detailed mechanisms of such IGF-independent actions of IGFBPs (Non-Patent References 9 and 10).